Cool Roofs Can Counteract the Warming of Urban Sprawl — Popular Mechanics

A new study finds that roofs painted with reflective material or planted with vegetation can offset the warming predicted to accompany urban expansion.

February 10, 2014 1:59 PM Text Size: A. A. A

Scientists predict that by the end of the century. U.S. urban and suburban areas will have expanded by an area about the size of South Dakota. That’s an awful lot of blacktop, adding to scientists’ worries that the growth of cities could increase global warming, in part because of the way a city absorbs heat. But a new study has found that cool roofs, those painted with reflective materials, and green roofs (those with plants on them) could counteract this warming.

"What we found for cool, green, and hybrid roofs is that they don’t just offset urban expansion&#151;they can offset additional warming," says lead study author Matei Georgescu. assistant professor in the school of geographical sciences and urban planning, and senior sustainability scientist at Arizona State University’s Global Institute of Sustainability.

Eco-friendly roofs have been in the news for a while. A few years ago, the federal government started an initiative to put them on its buildings. This new study, published today in the Proceedings of the National Academy of Sciences (PNAS), is one of the first to consider whether a major expansion of these roofs across the country could counteract some of the environmental negatives of urban sprawl.

Jonathan Foley. director of the Institute on the Environment at the University of Minnesota, tells PopMech via email: "This is a really important study because it shows how designing better urban systems (especially by changing rooftops) can help keep our cities cool as they grow (and otherwise have a stronger "urban heat island") and as global warming becomes more and more serious over time. While this has been considered for individual cities before, this study is really the first comprehensive look at how changing city rooftops could have widespread, positive impacts on climate across large metropolitan regions in the country."

Cities affect regional climates in two ways. The first and more familiar is through emissions of CO2 and other gases that warm the planet through the greenhouse effect. Greenhouse gas emissions are greater in cities, where car-driving, electricity-consuming people are concentrated. But cities also affect temperatures more directly because their blacktop and roofs trap heat. This is called the urban heat island effect. When you separate it from the climate effects of greenhouse gases, the researchers say in their study, the heat island effect "can be expected to raise near-surface temperatures 1 to 2 degrees Celsius, not just at the scale of individual cities but over large regional swaths of the country."

To counteract this effect, cool roofs (which include white roofs), are painted with a material that reflects light and heat instead of absorbing it. On the other hand, the plants that cover green roofs cool the air by using solar energy, which would otherwise heat the roof, to draw water through their vasculature and evaporate it from their leaves.

For the study, researchers combined data from an earlier Environmental Protection Agency study forecasting urban expansion with computer models for the way these changes, and adaptations such as green and cool roofs, can affect climate. (And it’s important to note that this was not a study of global climate change; it covered only U.S. cities and their surrounding areas.)

In the researchers’ models, cool roofs lowered temperatures more than green roofs. However, cool roofs also reduced precipitation in some areas, including Florida, which is a negative tradeoff. As Georgescu explains, precipitation depends on warm air rising and its moisture condensing and falling. By cooling the air near the earth’s surface, cool roofs reduce the supply of warm air. In California, on the other hand, where it rains little already, cool roofs would not significantly affect rainfall, so there’s less of a downside for using them there, Georgescu says.

"What we emphasize is that there are no one-size-fits-all solutions," Georgescu says, adding that different cities should use green and cool roofs in different ways depending on their unique geographical situations.

The authors emphasize that it’s important to consider more than just greenhouse gas emissions when trying to create "green" cities. "What’s touted so far as green cities are based on basically carbon, how much carbon was used to create this infrastructure," Georgescu says. "The physical impact of the urban heat island effect is just as important as the greenhouse effect."

Brian Stone. director of the Urban Climate Lab at Georgia Tech, agrees, saying that while emissions reductions are important, "there are many things you can do in cities that can also slow the pace of warming without reducing greenhouse gas emissions. It’s not the only tool we have for reducing warming."